1. Field of the Invention
The present invention concerns the technical sector of coating sheets of paper.
2. Description of the Background
In techniques for coating paper, cardboard or the like, a coating composition is known which may contain one or several inorganic fillers, one or several binders and various additives. The coating composition is then deposited on a support surface(s). Among these additives, the water-retaining agents are included, which are not to be confused with agents retaining fines or filler on wire.
The goal of coating is to improve certain physical and optical characteristics of paper, such as for example, its gloss, its brightness, its opacity, its capability for ink printing, its "smoothness" and other properties of great commercial importance.
A composition for coating paper is generally formed from a filler which may comprise one or several pigments, one or several polymer binders and various additives such as, especially, a lubricant such as calcium stearate, a wax, or a fatty acid ester, and possibly antifoaming agents, and the like, all of which is well-known to the person skilled in the art.
After deposition on the support, the coating color has a natural tendency to transfer onto the support all or part of the water and water-soluble material that it contains. It is necessary to control a too rapid migration which would harm the physical and optical characteristics of the coat. Therefore, retention of water in the composition or coating color is sought. Also, the water retention must be controlled to avoid a change in the rheology of the coating slip unused and recycled in the coating procedure.
Natural or synthetic water-retaining agents are usually employed such as carboxymethylcellulose (CMC), starch, polyvinyl alcohols (PVA) or even latex or emulsions of highly carboxylated polymers or polycarboxylates, for example, of the polyacrylate type. Such products are described, for example, in EP 0 509 878.
A specific class of retention agents is named as alkali-swellable polymers, and is described in the above-mentioned EP patent, and in U.S. Pat. No. 4,780,500 which will be explained below.
It is naturally essential that the various additives not degrade the other properties of the paper, such as its brightness, its opacity, its mechanical resistance, and the like.
The principal important optical characteristics of the paper are, among others, its opacity, its gloss and its brightness, particularly for printed supports.
In order to improve the brightness of a paper, additive, called "optical brighteners" are included in the coating composition, which reinforce the brightness effect of the sheet.
These optical brighteners are in a general and not limiting manner of the type that include in their molecule the stilbene component substituted by diamines and sulfonic groups. These brighteners convert part of the invisible UV radiation into radiation of the visible spectra, generally in the blue to violet range.
A nonlimiting example of a stilbene disulfonic acid derivative used as optical brightener is the product "Tinopal.TM.(SPP Z or ABP Z or SK)" marketed by the Ciba Company.
Another nonlimiting example of a known optical brightener is the product Blankophor.TM. (P or BPN)" marketed by the Bayer Company.
Usually the effectiveness of these products depends on their activation. "Activation" is understood here to mean the presence of a co-product that will allow the brightening effect to be revealed and principally to be amplified. And yet, it is known that certain water-retaining agents do not activate optical brighteners.
Moreover, it is known that certain products, i.e., the polyvinyl alcohols or CMC, have limited use because of the high viscosities they develop in coating colors in conjunction with their effectiveness for water retention. Thus, the papermaker is left with two properties that are eminently desirable to reinforce, but which most of the time are antagonists to each other.
For at least a decade, optimization of the effect has been sought with modem water-retention agents, such as thickening polymers, and that of activators of optical brighteners and especially PVA and other additives.
In EP 0 509 878, compositions are disclosed which "can be totally free of synthetic or natural thickeners," (page 2, line 52), which clearly reveals the disadvantages of these compositions in use.
U.S. Pat. No. 4,780,500 describes water-retention agents formed essentially of the acrylic acid monomer and secondarily of itaconic acid monomer, with molecular weight of 100,000-800,000. Neither of these two documents mentions the problem posed by the optical brighteners.
GB 1,467,127 mentions the possible use of PVA as binder in coating compositions, in the same amount as starch, casein, and the like. It mentions the possible presence of optical brighteners (page 2, line 10) but without posing the problem of their activation compared with water retention.
U.S. Pat. No. 3,793,244 describes water-retaining terpolymers of styrene, butadiene and itaconic acid and does not mention PVA.
GB 1,271,282 describes a mixture of latexes, one of which contains a high proportion of styrene. This mixture of latexes promotes water retention. However, no portion of the disclosure discloses the presence of PVA.
U.S. Pat. No. 3,687,884 describes a latex with a vinyl acetate/acrylic acid grafting.
U.S. Pat. Nos. 5,231,145 and 5,240,771 teach that it is possible, by grafting, to increase the amounts of PVA in the presence of copolymers with the condition that this copolymer is prepared by polymerizing it in the presence of PVA. The references point out, on the other hand, that simple mixing of this polymer with PVA does not enable a stable composition to be obtained. A need continues to exist for an polymer composition of improved water-retaining and optical brightener-activating characteristics for paper coating colors.